In terrestrial digital broadcast1services, a transmission band of one physical channel is divided into 13 segments for example. Therefore, signals of different modulation formats can be multiplexed in one physical channel. For example, 12 segments of 13 segments are transmitted in a 64QAM modulation format for fixed reception, and at the same time, one segment is transmitted in a QPSK modulation format for mobile reception. Therefore, it is possible to select a hierarchy to be received in accordance with a reception environment and a receiving terminal at the side of the receiving terminal. For example, in the 64QAM modulation format for fixed reception, since an information transmission rate is large, high-definition broadcast can be carried out. However, in the 64QAM modulation format, robustness to a change of a transmission path is weak and reception is difficult in a weak electric field. Therefore, reception is difficult in a harsh reception environment such as reception in a mobile body. On the other hand, in the QPSK modulation format for a mobile body, since an information transmission rate is small, QVGA using H264 compression, picture of about 15 fps, is transmitted. The picture quality is not so high. However, in the QPSK modulation format for a mobile body, a resistance to a change of a transmission path is high and reception can be carried out well even in a weak electric field. Therefore, the QPSK modulation format is thought to be used in, for example, a portable terminal or a vehicle-mounted receiving device. A hierarchy such as a hierarchy of 64QAM, in which robustness to a change of a transmission path is weak and reception is difficult in a weak electric field, is referred to as a weak hierarchy. A hierarchy such as a hierarchy of QPSK, in which a resistance to a change of a transmission path is strong and reception can be carried out even in a weak electric field, is referred to as a strong hierarchy.
In currently used digital broadcast reception devices for mobile bodies, in order to prevent disconnections of video and audio, the following measures are employed. That is to say, a reception state is determined based on values such as an electric field strength of a received broadcast signal, a block noise, BER (Bit Error Rate), and the like. Then, an optimum hierarchy is selected and received in accordance with the change of the reception state. This is referred to as a hierarchy switching. In this way, a digital broadcast reception method, in which a hierarchy is selected in accordance with the reception state by carrying out hierarchy switching, has been proposed.
FIG. 26 is a block diagram showing a configuration of a conventional vehicle-mounted digital broadcast reception device (hereinafter, abbreviated to “a vehicle-mounted receiving device”) (see, for example, patent document 1). Vehicle-mounted reception device 1100 shown in FIG. 26 includes antenna 1101, tuner 1102 for receiving a radio wave, and demodulator 1103 for demodulating the radio wave received by tuner 1102 into a digital signal. Furthermore, vehicle-mounted reception device 1100 includes TS (Transport Stream) processing device 1104 for separating various data included in the digital signal from the digital signal, decoder 1105 for converting the digital data from TS processing device 1104 into data such as video, audio and other data, and setting device 1108 for receiving an input necessary for operating vehicle-mounted reception device 1100. Furthermore, vehicle-mounted reception device 1100 includes selection device 1106 for taking into a predetermined signal output from tuner 1102 and the like, and selecting and outputting one of a plurality of display candidates included in the received signal; and outputting device 1107 for outputting multimedia data such as video and audio data, and the like, based on the data output from selection device 1106. Furthermore, vehicle-mounted reception device 1100 includes I/O device 1109 for receiving various input data of vehicle-mounted reception device 1100, ROM device 1110 and RAM device 1111 for storing a program necessary to start vehicle-mounted reception device 1100 and data, recording device 1112 for storing data necessary during operation of vehicle-mounted reception device 1100, and CPU 1113 for operating vehicle-mounted reception device 1100. The above-mentioned component elements are connected with bus 1161.
Furthermore, the below-mentioned signals are sent/received through connection lines 1152 to 1160 shown in the drawing, respectively. A RF (Radio Frequency) signal is transmitted through connection line 1152; and a digital data (for example, a transport stream) signal is transmitted through connection line 1153. Then, a digital data (for example, an elementary stream) signal is transmitted through connection lines 1154 and 1155. A video signal such as VGA (Video Graphics Array), NTSC (National Television Standards Committee), and PAL (Phase Alternation by Line), an audio signal such as AAC (Advanced Audio Coding), other signals such as a signal relating to text data, and the like, are transmitted through connection line 1156. Furthermore, a signal relating to an electric field strength showing the reception strength of the radio wave received by tuner 1102 is transmitted through connection line 1157. Then, a signal relating to BER (Bit Error Rate) showing a reception situation of data that have been demodulated into digital data is transmitted through connection line 1158. Furthermore, a digital signal for inputting or changing the display condition with respect to outputting device 1107 is transmitted through connection line 1159. A signal relating to an index showing a quality of video (for example, a block noise generation rate, a value of the total amount of motion vector, a disconnection degree of audio) is transmitted through connection line 1160.
However, since such a method carries out a hierarchy switching after it determines values such as an electric field strength, a block noise, and BER, it cannot respond to a rapid change of a reception environment, so that video and audio having a poor reception quality may be displayed before hierarchy stitching is carried out. In addition, during movement on a route whose reception environment is changed relatively drastically, hierarchy switching occurs frequently, which may make a user feel uncomfortable. In order to solve this problem, from the receivable area information, fixing to a strong hierarchy area for a while has been proposed in order to prevent excessive generation of switching before a hierarchy switching is performed frequently based on position information of a mobile body and receivable area information (see, for example, patent document 2).
As mentioned above, in the digital broadcast reception device in which the receiving hierarchy is switched in accordance with receiving states, since disconnections of video and audio occur for several seconds during switching of the receiving hierarchies, it is desirable that a switching frequency is reduced as much as possible. Thus, in a conventional digital broadcast reception device, in order to prevent too frequent switching, a technique for fixing broadcast to simple moving picture broadcast has been employed. However, since an appropriate switching holding time is different depending upon a running position and a receiving state, it has been difficult to constantly perform appropriate switching in various reception environments.
Furthermore, in vehicle-mounted digital broadcast reception devices, a diversity technology for individually demodulating and synthesizing input signals from a plurality of antennas is often used. Each system from a plurality of antennas is referred to as a branch. Digital broadcast reception devices having a two-branch configuration and a 4-branch configuration have been developed. The larger the number of branches is, the more the reception quality is improved and the error robustness is increased. Therefore, in a digital broadcast reception device having a plurality of branches, for example, a digital broadcast reception device having four branches, it is thought that three branches are used for receiving video and remaining one branch is used for channel scanning or obtaining programs. However, even in a position in which a satisfactory reception quality cannot be secured unless four branches are used, during processing using one branch in the background, only reception quality corresponding to three branches can be obtained. Even when deterioration of the reception quality is detected and reception can be changed to 4-branch reception, it is not possible to carry out a rapid response to the change of a reception environment.    [Patent Document 1] Japanese Patent Unexamined Publication No. 2005-277873    [Patent Document 2] Japanese Patent Unexamined Publication No. 2005-184219